In the production of semiconductor integrated circuits, lithographic exposure tools for reductively projecting and transferring a fine circuit pattern drawn in a photomask onto a wafer are extensively used. With the trend toward higher degrees of integration and higher functions in circuits, the circuits are becoming finer, and the lithographic exposure tools have come to be required to form a high-resolution circuit pattern image on a wafer surface with a deep focal depth. The wavelengths of exposure lights are becoming shorter. ArF excimer lasers (wavelength, 193 nm) are being used as the exposure light in place of the g-line (wavelength, 436 nm) and i-line (wavelength, 365 nm) heretofore in use.
Recently, the technique of immersion exposure (ArF immersion lithography) is known in which exposure with a lithographic exposure tool is conducted while filling the space between the projection lens of the lithographic exposure tool and the wafer with a liquid in order to attain a higher resolution with an ArF excimer laser. The shorter the exposure light wavelength and the larger the NA (numerical aperture) of the projection lens, the higher the resolution of the lithographic exposure tool. The resolution can be represented by the following expressions.Resolution=[k(process coefficient)×λ(exposure light wavelength)]/NANA=n×sin θ
In the expressions, n indicates the refractive index of the medium through which the exposure light passes. In ordinary techniques, n is 1.0 because the medium is the air. However, in the immersion exposure, pure water, which has an n of 1.44, is used as the medium and the lithographic exposure tool can hence attain an even higher resolution.
Furthermore, the technique of polarizing exposure is known in which polarized lights which exert an adverse influence on resolution are diminished to thereby increase image-forming contrast and improve resolution, in contrast to the exposure techniques heretofore in use which employ an exposure light composed of random polarized lights having various polarization directions.
In lithographic exposure tools for use in the immersion exposure, polarization exposure or general exposure to ArF excimer laser light, quartz, which is a birefringent crystalline material having a high transmittance and excellent resistance to laser lights, relatively inexpensive and easy to process, is attracting attention as a material for optical elements, e.g., optical elements concerning polarization, such as polarizers, depolarizers or wavelength plates, and optical diffraction elements such as diffraction lenses.
However, it has been ascertained that optical elements produced from quartz deteriorate in optical properties, e.g., light transmittance, when irradiated over long with a short-wavelength light having high output, such as an excimer laser light.
In order to overcome such a problem, patent document 1 discloses the following artificial quartz members (1) to (4).
(1) An artificial quartz member for use as an optical element to be irradiated with a pulsed laser light having a wavelength shorter than 1,600 nm and a pulse duration shorter than 100 ns, characterized in that when the quartz member is irradiated with 5.0×107 pulses of ArF excimer laser light having a fluence of 500 mJ/cm2, lowering of the refractive index of an ordinary ray or extraordinary ray generated at the irradiated part is 50 ppm or less.
(2) An artificial quartz member for use as an optical element to be irradiated with a pulsed laser light having a wavelength shorter than 1,600 nm and a pulse duration shorter than 100 ns, characterized in that when the quartz member is irradiated with 5.0×107 pulses of ArF excimer laser light having a fluence of 500 mJ/cm2, the height of the expanded irradiated part is 20 nm or less.
(3) An artificial quartz member for use as an optical element to be irradiated with a pulsed laser light having a wavelength shorter than 1,600 nm and a pulse duration shorter than 100 ns, characterized in that when the quartz member is irradiated with 5.0×107 pulses of ArF excimer laser light having a fluence of 500 mJ/cm2, the amount of induced birefringence is 90 nm/cm or less.
(4) An artificial quartz member for use as an optical element which transmits visible light and lights having shorter wavelengths than visible light, characterized in that the homogeneity in refractive index of an ordinary ray or extraordinary ray is 100 ppm or less.
However, (1) to (4) above merely show optical properties required of artificial quartz members to be used as an optical element, and it is not taught therein as to what artificial quartz member has such optical properties. Furthermore, no means for producing each of the artificial quartz members (1) to (4) is shown in patent document 1, and it is necessary to select an artificial quartz member having the above-mentioned optical properties from many artificial quartz members produced. The artificial quartz members which were not selected reduce the yield.
Patent Document 1: JP-A-2005-289693